Far NE Pannonian Basin – Trans-Carpathian Trough of Ukraine

The north-east part of the Pannonian Basin, actually its narrow band, is known in Ukraine as the Trans-Carpathian Trough. The region is located to the south-west from the Ukrainian Carpathians and is extensively eroded, with narrow ridges, steep (25-40o) slopes, narrow river and stream valleys, and altitudes of 400-800 to 1000 m. Mountain slopes are overgrown with beech forests, and at the watersheds Alpine meadows (polonyny) are developed.

Asymmetry of the mountains are a particular feature of the Ukrainian Carpathians with south-western slopes that are more gentle than the steeper north-eastern ones. In the Vygorlat-Gutynskiy Ridge that separates the Trans-Carpathian Trough from the Folded and Thrusted Carpathians, is a wide fore-mountain belt with heights between 220 and 450 m which gradually changes into the Trans-Carpathian low-land. This is a flat plain which is getting lower towards the Tysa River valley and is 100-120 m high. Some individual island-like mountains (Kholmets, Shalanky) or plateaus (Beregovo-Biganske) up to 300-368 m high are distinguished over the plain. And the plain is crossed by a dense river and drainage channel network.

The river network of the territory belongs to the basins of the Black Sea and Baltic Sea. The biggest river Tysa, the left branch of the Danube, flows from east to west in the southern part of the area and in some places it forms the natural border with Hungary and Romania. Its width is 80-150 m, depth – 2-3 to 6 m, flow speed – 0.6-0.9 m/s. The bottom is mainly sandy, the banks are abrupt (2-7 m high). The flood-land is covered by meadows. Over its entire course the river is fenced by a 2-5 m high dam.

Evidences for three major stages of geological development are encountered in the studied area: Proterozoic – Early Paleozoic (Baikalian or Asyntian), Paleozoic (Caledonian and Hercynian), and Mesozoic-Cenozoic (Alpine).

Palanok Castle of Mukachevo, west Trans-Carpathian Trough, built on the volcanic neck of Neogene andesite-basalts and porphyries (from: By alexjourba – alexjourba, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=5186951).

Development of the Trans-Carpathian Trough on the basement of Triassic-Paleogene, partly even Lower Miocene volcanogenic-carbonate-terrigenous sequences, started with the marginal fault and associated sub-parallel breaks in the inner depression part, where differential subsidence between fault blocks occurred. This is the young Late Alpine (Neogene) depression overprinting Internal Carpathian heterogeneous basement with a quite complex and not yet completely understood internal structure, where the Central Carpathian thrusts, extending from Slovakia and Poland, and structures of Maramures basin, extending from Romania, come together. According to most authors, the Trans-Carpathian trough is bounded by deep-seated faults: Trans-Carpathian in the north-east and Fore-Pannonian in the south-west. The Trough is filled with more than 3 km thick Neogene orogenic molasse sediments that form a terrigenous-volcanogenic complex. The external margin of this complex often extended outside the Trans-Carpathian deep-seated fault and can be found on the Internal Carpathian units (Peninska zone and Monastyretskiy thrust). Over there, above the Miocene molasse sedimentary sequence, a thick series of volcanogenic rocks is developed in the Vygorlat-Gutynskiy ridge, which is extended in the arc to the south-east and south to the border with Romania and which separates the Trough in two parts: the western (Chop-Mukachivska) and the eastern (Solotvynska) part. It was thought for a long time that these are separate tectonic zones or sub-zones with a different internal structure and geological history. Respectively, they were described as dimples with elevated molasse along the periphery. According to this traditional view, the Solotvynska depression has been set earlier and completed its development in the Miocene, while the Chop-Mukachivska depression has continued to develop in Pliocene and even in Quaternary times. Other authors suppose that the long-wise Trough zonation is of primary importance and they distinguish in the molasse (from the north-east to south-west): the Marginal (or Monocline) zone, the Central zone of salt-diapir and brachy-anticline folds, and the zone of Fore-Pannonian deep-seated fault. In the Marginal zone molasse rocks dip uniformly. The bed dipping angle from the Trough margins gradually decreases in the south-western direction from 30-40o (in places even 60-80o) to 15-5o and further horizontal bedding over a distance of 4-5 km. The thickness of the molasse sequences increases in the same direction. In the west the zone is overlain by volcanics of the Vygorlat-Gutynske ridge up to 1.5-2.6 km wide.

The Trans-Carpathian artesian basin is an area of slow groundwater flow and highly mineralized waters. In some areas of the Trans-Carpathian Trough and in the mountain river valleys groundwater is locally contaminated, resulting from sewers systems from animal farms, pesticides and other chemicals leaking from storage or reaching groundwater due to improper use, petroleum products and bitumens from storage tanks, near gasoline stations, or in relation to asphalt-bitumen plants, etc. For these reasons, in some places of Trans-Carpathian artesian basin, groundwater can containe elevated levels of metals, or can be enriched in nitrogen compounds like nitrates, nitrides and ammonia suggesting for extensive organic and bacterial contamination of the waters.

The region is also well-know with its morphological forms (some could be called geomanifestations) related to Neogene volcanic rocks developed in the western part around the towns Mukachevo and Beregovo. In the east, the outcrops of Upper Jurassic and Cretaceous sediments, as well as Neogene sediments and salt diapirs are known.

Boris MalyukGeoinform, Ukraine

For further diving into the geology of Ukraine, the relevant geological maps of Ukraine at scale 1:200 000 (in English, or accompanied by English translation):

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